This application claims the priority of Korean Patent Application No. 2002-59780, filed on Oct. 1, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a magnetic recording medium capable of recording data with a high density, and more particularly, to a magnetic recording medium having a magnetic recording layer which has a high thermal stability, high-density recording characteristics, and excellent SNR characteristics.
2. Description of the Related Art
Hard disk drives (HDDs), which are representative magnetic data storage devices and lead a rapid increase of the recording density, are currently adopting a longitudinal magnetic recording method where a ring-type head and longitudinal magnetic recording media are involved. A conventional longitudinal magnetic recording method, however, comes across a limit in increasing the recording density because of the thermal instability of a recording medium, and a new recording method, which is a perpendicular magnetic recording method, is currently being actively developed since the perpendicular magnetic recording method is expected to increase the recording density further well beyond 200 Gb/in2.
In contrast with an existing longitudinal magnetic recording method, in the perpendicular magnetic recording method, bits recorded on a recording medium are magnetized in a direction perpendicular to a substrate. The recording density can be further improved by using perpendicular magnetic recording media having the following characteristics: 1) a high perpendicular magnetic anisotropic energy constant Ku (>1×106 erg/cc) and a high coercive force; 2) small crystal grains; and 3) a low exchange coupling between magnetic particles.
General magnetic media having a single magnetic layer each include a recording layer and an underlayer. The recording layer stores magnetic information. The underlayer is formed on a substrate before the recording layer is formed, and improves the magnetic/crystal characteristics of the recording layer. General magnetic media having a double magnetic layer each further include a soft magnetic underlayer in addition to the recording layer and the underlayer so as to increase the intensity and spatial change rate of a magnetic field formed by a pole type recording head including an induction coil upon magnetic recording.
As shown in FIGS. 1 through 3, in conventional perpendicular magnetic recording media, perpendicular orientation underlayers 12 and 223 are placed below perpendicular magnetic recording layers 13 and 224, respectively, and a soft magnetic underlayer 22 is placed below a perpendicular magnetic recording layer 23. FIG. 1 shows a perpendicular magnetic recording medium having a single magnetic layer, and FIGS. 2 and 3 show perpendicular magnetic recording media each having double magnetic layers.
Referring to FIGS. 1 and 2, the perpendicular orientation underlayer 12 and the soft magnetic underlayer 22 are placed on substrates 11 and 21, respectively. The perpendicular magnetic recording layers 13 and 23 are formed on the perpendicular orientation underlayer 12 and the soft magnetic underlayer 22, respectively. Protection layers 14 and 24 are placed on the perpendicular magnetic recording layers 13 and 23, respectively. Lubricating layers 15 and 25 are formed on the protection layers 14 and 24, respectively, to protect the protection layers 14 and 24 and the perpendicular magnetic recording layers 13 and 23 against collisions with a data writing/reading head slider and induce smooth sliding of the data writing/reading head slider.
Compared to the perpendicular magnetic recording medium of FIG. 2, the conventional perpendicular magnetic recording medium of FIG. 3 further includes the perpendicular orientation underlayer 223 between the perpendicular magnetic recording layer 224 and the soft magnetic underlayer 222.
It is known that magnetic properties of a CoCrPtX-based (X=B, Nb, Ta, or O) alloy thin film, which is widely used to form a perpendicular magnetic recording layer 13, 23, or 224, are greatly affected by the type and structure of an underlayer.
Ti, which has been widely used to form a perpendicular orientation underlayer, is known to form a thick initial growth layer due to a relatively large difference in a crystal lattice constant between Ti and a CoCrPtX-based alloy thin film for a perpendicular magnetic recording layer, thereby degrading the orientation characteristics of the perpendicular magnetic recording layer.
Pt, which can be used to form a perpendicular orientation underlayer, makes a perpendicular magnetic recording layer have excellent perpendicular orientation characteristics because of a small difference in the lattice constant between Pt and a CoCrPtX-based alloy for a perpendicular magnetic recording layer. However, Pt increases the sizes of the crystal grains of the perpendicular magnetic recording layer made of a CoCrPtX-based alloy (particularly, a CoCrPtX-based alloy containing 10 or higher atomic percent of Pt) and also significantly increases an exchange coupling between magnetic particles, thereby reducing the signal to noise ratio (SNR) upon data writing/reading. A degree to which the use of the Pt underlayer increases the size of the crystal grains of the recording layer and the exchange coupling between magnetic particles is closely related to the thickness of the Pt underlayer. If a thick Pt underlayer is used, as described above, a high perpendicular magnetic anisotropy (Ku) and a high coercive force are obtained due to the excellent perpendicular crystal orientation of the recording layer. However, the enlargement of the crystal grains of the Pt underlayer increases the sizes of the crystal grains of the recording layer and the exchange coupling between magnetic particles. On the other hand, if a thin Pt underlayer is used, the sizes of the crystal grains of a perpendicular magnetic recording layer and the exchange coupling between magnetic particles do not increase much. However, the degree of the perpendicular orientation of the perpendicular magnetic recording layer decreases, thereby providing a low perpendicular magnetic anisotropy energy constant Ku and a low coercive force.